1. Technical Field
The present invention relates to optical gain media and, more particularly, to solid state gain media such as an optical fiber amplifier. As used herein, solid state gain medium is in reference to an optical fiber amplifier or laser.
2. Background Art
Optical fiber amplifiers receive coherent light of relatively low power from laser injection sources and amplify the light to higher power. Such amplifiers have been used in fiberoptic telecommunications and cable television systems to boost the power of a modulated optical signal being transmitted along a fiber transmission line. Erbium-doped fiber amplifiers (EDFAs) have been found to be especially convenient for fiber communication systems because their amplification wavelength (near 1.54 .mu.m) is conducive to low loss propagation of optical signals in glass transmission fibers. Various U.S. patents describe such systems, and the fiber amplifiers used by them, including but not limited to U.S. Pat. No. 5,185,826 (Delavaux); U.S. Pat. No. 5,218,608 (Aoki); U.S. Pat. No. 5,218,665 (Grasso et al.); U.S. Pat. No. 5,331,449 (Huber et al.); U.S. Pat. No. 5,337,175 (Ohnsorge et al.) and U.S. Pat. No. 5,339,183 (Suzuki). Various forms of signal modulation are used in these systems. Laser diode signal sources are capable of providing 10 mW to 100 mW, single mode, modulated light beams at high modulation rates, typically greater than 10 MHz, with low modulated drive currents. The modulated signal can then be amplified to higher powers, most typically up to about 100 mW, via the fiber amplifier.
An object of the present invention is to provide an apparatus for pumping a gain medium that produces adequate gain over a range of operating temperatures despite wavelength shifts in the pump source due to changes in the ambient temperature of the pump source.
According to this invention, an apparatus for pumping an optical gain medium, such as a fiber gain medium, comprises a pump source having a plurality of different spatially seperate or multiple wavelengths or wavelength bands which are all coupled into the fiber gain medium and provide at least one or more wavelengths to fall within the absorption band of fiber gain medium wavelengths due to changes in operating temperature of the pump source. As the multi-wavelength pump source changes wavelength with temperature, such as in a range within about -15.degree. C. to about 125.degree. C., at least one or more of the pump wavelengths will sufficiently overlap the gain spectrum of the fiber gain medium to continually provide high input power for pumping of the fiber gain medium. The invention herein provides for pumping sources for amplifier systems of the various disclosed embodiments disclosed herein resulting in high optical power pumping in spite of changes in operating temperature and corresponding wavelength shift in the operation of the pump source.